Thermal printheads are known which are laminated structures comprising an alumina substrate having alternating conductive and insulating layers (see for instance, U.S. Pat. No. 4,651,168 to Terajima et al.). Such prior art printheads, as illustrated in FIGS. 1A through 1E typically comprise an alumina substrate (10) having a metallic layer disposed thereon which may be patterned to provide a plurality of selectable electrodes (12). An insulating layer (14) of glaze is usually disposed upon the selectable electrodes (12) and subsequently has disposed thereon another metallic layer which provides a common electrode (16). A protective insulating material (17) may be disposed on the common electrode (16). The depth or amount of insulating glaze (14) disposed on the plurality of selectable electrodes (12) typically determines the length of heating elements or thin film resistors (18) disposed between respective selectable electrodes (12) and the common electrode (16).
Print quality is effectively a function of the resistors (18) and the characteristics of the insulative layer (14) upon which the resistors (18) are disposed. Certain characteristics of the resistors (18), such as the length determined by the insulative layer (14), significantly influence print quality, especially in long, high resolution printheads. Width of the resistors (18) is also a critical characteristic, because resistance value of a particular resistor is determined by first dividing the length of the resistor by its width to determine a number of "squares" of resistive material. The number of squares is then multiplied by the sheet resistance (Ohms per square) of the particular resistive material to determine the total resistance of each resistor. Total resistance determines the amount of heat generated for thermal printing. Thus, the length and width, i.e. resistance, of these resistors (18) must be accurately controlled to achieve high quality printing.
Ideally uniform print quality from resistor to resistor would require, as illustrated in FIG. 1A, an ideally uniformly planar substrate (10), perfectly regularly shaped selectable electrode (12) geometries, an ideally uniformly applied insulative layer (14), and an ideally uniformly planar second metallic or common electrode layer (16). However, as illustrated somewhat exaggeratedly in FIGS. 1B through 1E, various imperfections and irregularities occur in the fabrication of such laminated edge-type thermal printheads. Imperfections and irregularities effect resistor dimensioning, ultimately negatively impacting print quality.
Imperfections or unevenness in the alumina substrate (10), as illustrated in FIG. 1B, may be perpetuated throughout the various layers of the printhead. An uneven substrate (10) results in subsequently applied uneven and irregular selectable electrodes (12). Further, a similarly unevenly applied insulative glaze layer (14) will be disposed upon the electrodes (12) and substrate (10) and result in a correspondingly uneven common electrode layer (16).
Significantly costly mechanical processes may be undertaken, such as lapping and polishing of the substrate (10) to assure an even substrate (10), such as illustrated in FIG. 1C. However, lapping and polishing of the substrate (10) will not assure precision etched electrodes (12). Standard photolithographic techniques may not be adequate to uniformly meet the dimensional requirements of an electrode thickness in the order of 5 microns, necessary to achieve good electrical connection to the resistor and may result in irregularly shaped electrodes. Further, close spacings of electrodes (10-15 microns). necessary in high resolution (greater than 200 dpi) heads and required for complete electrode/resistor contact, are difficult to achieve with standard photolithographic techniques because of increased likelihood of bridging and shorting. The resulting electrodes, overetched to reduce the likelihood of shorts, may lack full resistor contact, such as illustrated in FIGS. 1C and 1D. Full dot width printing may be precluded because current from the electrode (12) will not spread adequately throughout the resistor to heat the entire resistor surface area. Thus, a precision etching technique, such as ion milling, would be necessary to make the widest possible electrodes with narrow spacing between them as required for high resolution heads.
However, precision etching techniques add additional and expensive processing steps and cannot absolutely preclude bridging and shorting between electrodes that may result never-the-less from lumpy, high granularity etchable thick film gold paste used in the electrode fabrication process. Greater precision and quality may require highly refined pastes.
Although precision ion milling of the selectable electrodes fabricated from highly refined pastes, permits greater control of the electrode geometry that can be fabricated on a precision ground or lapped substrate, resistor length and consequently print quality may still be negatively impacted by application of a non-uniform insulative glaze layer (14), such as illustrated in FIG. 1E. Elimination of imperfections in the insulative layer further requires surface finishing, such as precision grinding or lapping in order to avoid irregularities resulting from laminating the common electrode (16) on top of insulative layer (14) imperfections. Precision grinding or lapping of the insulative layer must also be highly controlled so as to avoid irregularities in the polished insulative layer, such as a wedged, uneven grinding as illustrated in FIG. 1F.